This invention relates to direct reading dosimeters and more particularly to solid-state direct reading dosimeters which measure neutron and gamma radiation.
A requirement exists for military forces to measure combined dose from neutron and gamma radiation independent of the dose rate. The only device currently available which will meet this requirement is the 1M185, an evacuated quartz-fibre pen dosimeter, developed by the US Army.
The problem associated with this prior art device is that it is relatively expensive and can only be used in conjunction with its charging unit. The dosimeter must be recharged to compensate for loss of electrical charge and pumped to ensure an adequate vacuum in the chamber.
U.S. Pat. No. 4,484,076 which issued Nov. 20, 1984 to Ian Thompson discloses a direct reading dosimeter for use by personnel workers in the medical, nuclear and industrial field. This dosimeter provides an indication of dose rate as well as total received dosage. This prior art dosimeter will, however, only measure gamma radiation. This dosimeter utilizes a single commercial type MOSFET. These MOSFET's offer a sensitivity in the range of 0.1-0.2 mV/rad (mV/cGy). The operating gate voltage would therefore have to be balanced to within 1 or 2 mV and adjusted for each device for an operating gate-to-source voltage of 4 to 6 volts. Long term drift is fairly common for these type of circuits, accordingly an undesirable periodic adjustment of the gate voltage would be required.
The change in gate voltage, for a specified operating drain current, with a change in temperature of only a few degrees could be several millivolts and would accordingly affect the accuracy of the measured dosage.
It is therefore an object of the present invention to provide a solid state dosimeter which will measure gamma and neutron radiation.
Another object of the present invention is to provide a solid state dosimeter having an increased sensitivity.
Yet another object of the present invention is to provide a solid state dosimeter which is relatively insensitive to temperature changes and requires no operator adjustments.